CIRCULATION OF THE SAP. 123 



part of the stem. If the same tree be cut in mid-summer, there 

 will be little or no issue from the alburnum, but the bark will 

 now give out a fluid from the upper edge of the wound, prov- 

 ing that the downward current is through the bark. The 

 reason that has been assigned for little or no sap issuing from 

 the cut alburnum in summer is, that the draft made upon it 

 by the evaporation prevents the vessels from holding enough 

 sap to issue from the cut ends. 



226. That the sap, before elaboration, ascends within the 

 wood, and that most of it, after this process, descends within 

 the bark, is proved by tying a ligature very tight round a 

 branch in spring, and the branch will greatly increase above 

 the ligature, and but very little below it, thus showing that the 

 sap was not obstructed in its ascent, but was obstructed in its 

 descent. This operation will very much increase the size of 

 fruit on any branch for a single year, but it injures the tree for 

 succeeding years, since the proper amount of alburnum is not 

 deposited in the trunk, and from the hardening of the previous 

 alburnum, the sap for the succeeding year is obstructed in its 

 course. 



227. The cause of the ascent of the sap has been attributed 

 to the evaporation of the leaves, to capillar!/ attraction, aided 

 by the motion of the stem produced by the wind, to endosmose, 

 and to vital action. We believe it is generally not due to any 

 one of these, but to all of them, and we believe more is due to 

 vital action than to any other cause. That it was wholly 

 owing to vital action in the first experiment quoted from Hales, 

 is evident from the fact that none of the other alleged causes 

 could act. Evaporation from the leaves could not have pro- 

 duced it, for he states that there were no branches on the stem 

 subjected to experiment. Evaporation or Endosmose cannot 

 produce a force exterior to the body in which they act. To 

 vital action alone, then, we must ascribe the principal force 

 with which the sap is propelled. It seems remarkable that so 

 much pains should be taken to explain phenomena on mechani- 

 cal principles, which are wholly impotent when applied to the 

 circumstances under consideration. We are gravely told, and 

 I quote high authority, that "when a young bud is first excited 

 to growth in the spring, the fluids it contains are increased in 

 density by evaporation; endosmose immediately takes place 

 between it and the tissue -below it, which latter parts with the 

 thinnest portion of its contents, and then acts by endosmose 



"Why does not the sap flow in summer ? — 226. How is the direction of 

 the sap proved? What effect on the fruit?— 227. To what causes has the 

 ascent of the sap been attributed ? What do Hales' experiments prove ? 



